In testing and treating water for drinking purposes, it is necessary to test the water's turbidity. Turbidity has a marked effect on the bacteriological quality of water, whether or not disinfection is practiced. This is so because turbidity interferes with the ability to disinfect water. As recited in the Federal Register, Vol. 40, No. 248, published on Wednesday, Dec. 24, 1975, the maximum contaminant levels for turbidity were outlined as:
(a) one turbidity unit (NTU), as determined by a monthly average pursuant to section 141.22, except that five or fewer turbidity units may be allowed if the supplier of water can demonstrate to the state that the higher turbidity does not do any of the following:
(1) interfere with disinfections; PA1 (2) prevent maintenance of an effective disinfectant agent throughout the distribution system; or PA1 (3) interfere with microbiological determinations.
(b) Five turbidity units based on an average for two consecutive days pursuant to section 141.22.
Turbidity is measured by use of a turbidimeter which includes a light source for illuminating a sample to be tested and one or more photoelectric detectors with a readout to indicate the intensity of light scattered at right angles to the path of the incident light. The greater the scatter, the greater the turbidity. In testing for turbidity, a turbidity reference suspension must be selected which is readily reproducible and which can be used to calibrate the turbidimeter. Until the present invention, no such reference existed.
Prior attempts to measure turbidity were, in retrospect, rather crude. FIG. 1 shows the first accepted means of filling cylindrical container 1 with water 3 to a height 5. Candle 2 was placed at the bottom of the transparent cylindrical vessel. The illumination produced by the candle was viewed at the water surface 4. Simply stated, water level 5 was increased until the candle's illumination could no longer be seen at 4. The technique was proposed by Jackson and the height of the water was read in Jackson Turbidity Units (JTU). The result was a crude determination of the turbidity of water for as water became more turbid, the height 5 became smaller for a given sample.
The Jackson method can only be described as primative at best. The candle would blacken the bottom of the transparent vessel thus interfering with the pure turbidity measurement. Furthermore, sedimentation would precipitate out of solution and would block the candle's illumination, although such sedimentation has nothing to do with turbidity.
The next advance in turbidity measurement involved the use of formazin suspended in water as the reference. Formazin is the condensation polymer of hydrazine sulfate (NH.sub.2).sub.2 H.sub.2 SO.sub.4 and hexamethylenetetramine C.sub.6 H.sub.12 N.sub.4. Unfortunately, hydrazine compounds are extremely toxic and their use as a preparation of a turbidity standard for water represents certain disposal problems and health problems which should be avoided if possible. A further drawback to the use of formazin as a standard is that in the 1.0 and 5.0 NTU range, a non-linear dilution of formazin concentrate is necessary for, in such low concentrations, formazin decomposes. Formazin is prepared via a standard condensation reaction: ##STR1## As the formazin solution becomes more dilute, the reaction is pushed to the left, thus breaking down the compound.
A further drawback experienced through the use of formazin is that it characteristically has a sedimentary light scatter loss of approximately 10% for four hours. This means that care must be given to thoroughly mix any formazin suspension prior to sampling. Such a problem can be more readily appreciated by viewing FIG. 2. The turbidity of a formazin containing standard is done by illuminating transparent cylinder 7 through its base by light source 9. The formazin 15 suspended in water 6 causes the light to scatter and a light reading is taken at right angles to the incidence of illumination at 8. The turbidity units are known as Formazin Turbidity Units (FTU). Again, the greater the turbidity, the greater the light scatter.
Because of the problems outlined herein, a formazin standard, although in use for over 40 years, is not at all accurate. The sedimentary light scatter loss is significant and, as was true with regard to the Jackson turbidity test, sediment acts to block the illumination source and results in false readings of the standard. Lastly, formazin diluted has a life expectancy of approximately one week while in a concentrated form its life expectancy is approximately 30 days.
It is thus an object of the present invention to eliminate the drawbacks as outlined above.
It is a further object of the present invention to provide a polymeric standard which can be used as a reference suspension for determining the turbidity of water without any of the drawbacks experienced when using formazin.
It is yet another object of the present invention to prepare a polymeric material which can be used as a standard reference suspension having a shelf life much longer than materials used for the identical purpose in the past.
It is yet another object of the present invention to prepare a polymeric material useful as a standard reference suspension in the measurement of turbidity in water which is non-toxic.
It is yet a further object of the present invention to produce a polymeric material useful as a standard reference suspension in measuring the turbidity of water which is stable at extremely low concentrations.
It is still another object of the present invention to produce a polymeric material useful as a standard reference suspension to measure turbidity in water which has a substantially lower loss in light scatter due to sedimentation than formazin and other prior art materials.